These reagents and compositions are mixtures of lime hydrate and/or calcium hydroxide with one or more additives and are characterized by an especially large specific surface area and/or an especially small particle size.
Lime hydrates with a high specific surface area of up to 40 m.sup.2 /g are already known and yield higher separation results in purification of exhaust gases in comparison with the conventional commercial lime hydrates. These lime hydrates are produced by slaking lime with the help of alcohol. They have the great disadvantage that production is extremely complicated and expensive, and despite the recovery of the alcohol, a residual alcohol content remains in the product in amounts of several percent.
Lime hydrates for purification of exhaust gas are known from the literature and from German patent application publication 3,716,566. These known lime hydrates have basic, oxidizing, reducing, hydrate-forming or adsorptive properties or heavy metal binding properties. These additives are used exclusively to achieve a high reactivity, for example, in exhaust gas desulfurizing plants, without taking into account the effects of these additives on the residues from such desulfurization plants obtained after these processes. Consequently, these residues are not safe for dumping due to the leachability of certain additives. In particular, the original surface area of the mixture or components of these known mixtures is not affected by additional process measures in production.
German patent application publications 3,826,971 and 3,915,934 disclose lime products for purifying gases and exhaust gases. These lime products are produced by adding surface-active substances such as activated carbon and lignite coke or blast furnace coke, activated aluminum oxide or silica gel, to the lime or the slaking water in the production of said lime products and also adding catalytically active heavy metals such as vanadium and also heavy metal binding substances such as sodium sulfide. These products pose problems in final storage in residue dumping sites because apart from their general leachability, even when catalytically active heavy metals and heavy metal oxides are added they can also be leached out, which is especially undesirable in residual products that are to be dumped.
For example, it has been found that when activated carbon alone is added to lime hydrate, for example, binding of mercury is improved, but then substantial levels of mercury are found in the eluates of the residues.
Adding active carbon also has the great disadvantage that even when amounts of only 2 to 5% are added to the total mass of the composition, the carbon content is high enough that spontaneous ignition can occur, especially when there is an irregular distribution or pockets of high concentration are formed. There have already been several instances of this, leading to smoldering fires in silos holding such residues.
Furthermore, processes for purifying exhaust gases are known from the literature where an activated carbon reactor downstream from the main purification stage is used. There is the danger of fires also in this traditional system. In order to overcome this problem, the temperature of the flue gases would have to be lowered accordingly, which would in turn be associated with an additional energy consumption for reheating before the flue.